ON PASTURE.COM – Preventing animals from taking that “second bite” or regrazing an individual plant before it has time to recover is a basic tenet of managed grazing. It’s the reason many use adaptive, multipaddock grazing both to prevent regrazing and to increase production of grasses that might be harmed most by that second bite.
But what if our assumptions about regrazing and how rotational grazing prevents it are wrong? That’s the question that stakeholders asked researchers to explore as part of the Collaborative Adaptive Rangeland Management experiment (CARM). The unexpected result might have you rethinking how you manage your own pastures.
What makes this experiment different?
The Collaborative Adaptive Rangeland Management Experiment (CARM) is different because it tests real management practices, at a ranch scale. Eleven stakeholders, including ranchers, researchers, state and federal land managers, and staff from environmental conservation organizations set goals and objectives and make decisions to manage grazing for healthy vegetation and wildlife habitat as well as profitable ranching.
Stakeholders are particularly interested in comparing adaptive rotational grazing to traditional management (season-long grazing from early-May to early-October). So, they set up a system of ten pairs of 320-acre pastures similar in size, soil, and vegetation. In one set of ten pastures, stakeholders grouped steers into one large herd and rotated them among eight of the pastures, with two set aside for planned rest as a drought management strategy and to meet vegetation and wildlife objectives. These pastures are labeled as CARM for Collaborative Adaptive Rangeland Management. In the other set of ten pastures, the same total number of animals are split into ten small herds that graze season-long, creating the same ranch-scale stocking rate as the rotationally grazed pastures. These season-long pastures are labeled as TRM for Traditional Rangeland Management. Since the project began in 2012, researchers have been monitoring and measuring outcomes, and stakeholders have been adjusting management based on that information.
Western Wheatgrass and Regrazing
Cool season (C3) grasses are important because they provide early season forage for grazing.
Western wheatgrass is a cool season (C3) grass and stakeholders would like to have more of it in their pastures. So, they asked researchers to set up a study that would help them find out if their management was preventing cattle from regrazing and reducing the grass’s health.
Examples of how tillers were individually marked. Note the white tips that show how grazed tiller tips were marked with white paint to distinguish regrazing from initial grazing. Figure 1 from L.M. Porensky et al, Collaborative Adaptive Rangeland Management, Multipaddock Rotational Grazing, and the Story of the Regrazed Grass Plant, Rangeland Ecology & Management 78 (2021) 127-141.
The study team set up 40 monitoring plots – four in each pasture. Then, before the grazing season began, they used plastic twist ties placed at ground level to mark western wheatgrass tillers along a transect in each of 3 plots per pasture (30 tiller transects total). Since this was a two-year experiment, different tillers were marked in year two. These tillers were monitored and measured repeatedly through the growing/grazing season. Adaptively managed pastures were counted and measured twice per week while the large herd was in the pasture. Then they were measured again once per month when cattle were no longer in the pasture to capture any grazing by wildlife. In the continuously grazed, TRM pastures, tillers were monitored every other week.
But this description of the sampling process doesn’t do justice to the hard work being done. Imagine 15 research assistants crawling around on the shortgrass prairie wrapping tillers with plastic twist ties; then crawling around again, week after week, to count grazed and ungrazed tillers and paint the tiny tops of the grazed ones with a little white paint so they could track whether a tiller was grazed twice. It takes a lot of dedication to do this kind of detailed work.
What did the data say?
There were a number of take homes from this study, some expected, some unexpected. Let’s take them one by one.
Regrazing is bad.
Regrazed tillers, or those that had only been grazed once, were shorter than their ungrazed counterparts, reinforcing this long-time lesson.
Staying in a pasture longer or being in a pasture early in the season when the species is most palatable, DOES lead to more grazing and regrazing of tillers.
In this study, pastures that were grazed early in the season by the CARM treatment with higher stock density showed more regrazing than either the CARM pastures grazed later, or the TRM continuously grazed pastures.
The surprise here is the grazing system (CARM vs TRM) had little influence on the frequency or intensity of grazing at the ranch scale. In fact, the results of Traditional Range Management are very similar to the adaptively managed results as you can see below in this ranch-level summary of tiller regrazing. Researchers and stakeholders were surprised to find out that in the TRM pastures, at moderate stocking rates, leaving cattle in one place does NOT mean that all tillers of a palatable species get grazed. In fact, roughly 2/3 of the tillers never got grazed, even under continuous season-long stocking.
Figure 3 from L.M. Porensky et al, Collaborative Adaptive Rangeland Management, Multipaddock Rotational Grazing, and the Story of the Regrazed Grass Plant, Rangeland Ecology & Management 78 (2021) 127-141.
Season of use did make a difference as the graphs below illustrate. In the CARM treatment, some pastures got grazed quite heavily, but this was offset by other pastures that got grazed very lightly. In contrast, TRM pastures were grazed more evenly across the board. Those pastures where more regrazing occurred were during the early season when western wheatgrass was more palatable, and stock density (more animals for a short time) was higher.
Figure 5. Proportion of western wheatgrass tillers grazed or regrazed in each of the 10 replicate pastures in each treatment in 2017 for adaptive multipaddock rotational grazing treatment (CARM) Traditional Rangeland Management (TRM). Treatment labels Pastures are paired into 10 blocks, which are sorted on the basis of the order in which CARM pastures were grazed (left-most CARM pasture was grazed first). For reference, dashed black lines show the percent of tillers that remained ungrazed in the light and heavy treatments.
The most important take home message
Stocking rate is the major driver of behind how many plants are regrazed, NOT the grazing system.
If you notice, each of the graphs above include a block for “light” grazing and one for “heavy grazing.” These represent data from pastures included in a study that has been ongoing since 1939 comparing a light stocking rate (20% utilization) and a heavy stocking rate (60% utilization).
When you look at the data and compare the results of the heavily stocked pasture to the moderately stocked CARM and TRM pastures, you see what happens when stocking rates increase. If you have too many animals grazing, you’re going to run into regrazing no matter what you do. In contrast, season-long rest is “built-in” for ranches stocked at light-to-moderate rates, regardless of grazing system.
Even if managers adaptively managed this heavy stocking by rotating them through pastures, nothing would change. As Dr. Porensky pointed out when we spoke, at a ranch scale, it’s a zero-sum game. Less grazing in one pasture leads to more grazing elsewhere. As a result, rotation doesn’t affect the total amount of grazing and regrazing across all pastures.
Simply rotating without attention to stocking rate is NOT a silver bullet.
Here’s what the authors of the paper conclude:
“We found that CARM grazing did not lower rates of grazing and regrazing on western wheatgrass tillers at the ranch scale in the shortgrass steppe, when compared with season-long continuous grazing at the same stocking rate. Thus, the use of adaptive multipaddock rotational grazing strategies should not be expected to enhance abundance of this palatable, cool-season species. In fact, when viewing the world form the humble perspective of a western wheatgrass tiller, it is apparent that season-long rest is built into season-long continuous grazing and rotational systems. More than 50 years ago Morris (1969) wisely summarized that “under the system of ‘correctly [moderately] stocked’ continuous grazing, described here, plants were grazed neither frequently nor severely…Individual small areas in these ‘continuously grazed’ swards were thus grazed rotationally.”
In the end, like many previous studies, we conclude that stocking rate is a far more important driver of ranch-scale defoliation intensity and frequency than…movement of cattle among paddocks.”
The authors also point out that this does not mean that adaptive rotational grazing is not a valuable management tool. It could be very useful for achieving objectives specific to individual pastures. For example, managers could use this information to minimize or maximize impacts on western wheatgrass or on other species, or as a means for improving wildlife habitat, or for changing populations of palatable species within a pasture. The thing to remember is that any decision to decrease use of one pasture will lead to increased use of other pastures, unless ranchers utilize supplemental feeding or off-ranch forage.
Thinking about it
A good question for Thinking Graziers is “Why is it that the grazing system is not more important in this case?”
Dr. Porensky had this same question and has some ideas about the answer:
Cattle shift diet preferences as plant palatability changes through the season. Other studies suggest that plant height and forage quality can influence regrazing. When grazing on the shortgrass prairie, animals get more per bite from longer, ungrazed plants. In addition, western wheatgrass declines in quality as the season progresses, and other forages become more palatable, so animals choose them instead. In addition, in the shortgrass system, regrowth depends on rain that may or may not come, limiting palatable regrowth. This all combines to mean less regrazing of western wheatgrass tillers.
It also means, in this system, regrazing to keep plants in the vegetative stage is not likely to work as well as it does in systems with lots of precipitation and lots of plant regrowth. This is a really important point when thinking about your grazing management. If you’re in an area with lots of precipitation, and lots of regrowth there is more potential for regrazing and more potential benefit from rotation. Still, the researchers caution that “several studies in mesic systems have failed to document strong effects of grazing system on levels of selectivity or regrazing rates at the plant scale. Further research is needed to identify the specific conditions under which rotation may influence regrazing frequencies.” (page 137 of the paper.)
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